Electrically operated overhead traveling crane



y 1950 KAl-KOBAD s. KUKA 2,509,153

\ELECTRICALLY OPERATED OVERHEAD TRAVELING CRANE Filed May 21, 1948 CAPACITOR "QONTROL SWITCH Patented May 23, 1950 UNITED STATES OFFICE.

ELECTRICALLY OPERATED OVERHEAD TRAVELING CRANE Kai-Kobad' S. Kuka, Jamshe'dpur; India, assignor toz l heTata lron &; Steel Co. Ltdi,..Jamshedp ur,..

India Application May 21, 1948, SerialNo; 28,414 In India February-; 1 948 loads, or-dynamic braking-torque when. lowering 5 heavy. overhauling. loads. The. electrical. control should also have. maximumv flexibility. so that. it. can; be. easily adjusted: to suit. the requirements. of: the-craneeon whichit. is; installed.

For thewhoisting duty, the. controller connects. the. motor to. the. power lines with the. motorarmature, the motor seriesfield, the operating. series; coil of the. electro-mechanical brake and the. speed accelerating resistances in. an. elec. trically series connected. circuit. Increasing. values: of;spe.ed. and torque areobtained by de-- creasing; the amount of. resistance.- in. series with. the motor. On. thefinalpoint the. motor gets. connected directly acrosssthe-power'lines as: a. straight. series; connected: motor.

For the lowering, duty; the connections. aresoi made that the motor is. converted into a shunt.- motor by putting the motor seriesfieldacross the; line withresistance. in; series.- connection. and the. whole. connected parallel. with the. armature Gill-'- cuit, i-. e., during lowering theseriesfieldcircuit andthe. armature circuit are in parallel.

The .seriescoil of the electromechanical brake: is always connected .in-serieslwith-the motor field i. e., while:lowering,.the-brakecoil always. comes. in the series field circuit. When thus connected, the motor develops a motoring torque if light hook loadis i to bewhandled; ordynamic braking torque if. heavyloads areltolbe. lowered; The changesfrom power-.tddynamic braking andvice: versa. in; the lowering direction 183211 inherent feature of the hoistmagneticrdynamic.braking; crane controllers, and, is independent. of either the operator Or-the functioning of the control devices.

Thezspeeds are controlledcover amaster switch:v which has normally-1 five. control points; both: in: the hoisting-- and: the: lowering directions; The master switch operatesthe various: contactors in. the control panel, which their vary. the resist ancesprovided in themotor-armature, the motorseries field and the -power=lihe circuits; The-acceleration and decelerationdur-ing lowering as: well as hoisting cycles are automaticallyp'or"-- vi'd'ed underthe-control ofmagneticdefinite tiine 2'. relays. The controller has additional safety-features,. viz., main. disconnecting lines knife switches, oyerloadxrelays; controls.knife switches: and. control: circuit: fuses. lnzthe; final. notch: oi:

the master switchzduring lowering,.the;motor-has:-

maximunu resistancez in the series. field.- circuit,

so" as to :get'minimumpossible.motor field current. to get. maximum. lowering speeds; both. with light;

hook. or heavy loads;

A. direct; current series; wound: millityper motor,, because. of favourable torque: characteristics:

is .standar disedlfor-rcrane service,.when thecranes;

With weakened: field,. the. mill: type series: motor:

develops less torquezwhen it;is working as a gen.- erator to: create dynamic. braking action, than when: it? works. as .a. motor at that; current.

This is due to the:- effect ofi theLarmaturereac-- tion. athigh: armature currents. with weakened The; armature reactiontends" to weaken the field: in. case of. generator action. Thus am unstable action is set in, whichhecomes cumula tive when once started, causingt-he motor-torun away at enormous speeds;

Due to the above.inherentxstability limitatiom the fieldcurrent during: lowering for ahoistconz troller is never reduced: tobelow-'50 per cent of? its normal. full: load current rating; If. the field currentiis below. 501per cent, themotor operates very near its. stability. limit .while. handling: heavy loads'at high speeds with reduced fields. The limitation of'the field weakening at heavy load, therefore gives averagespeed' diff'erenceof about 25 per cent between the maximum light hook lowering speed and maximum full load lowering speed, thelight load speed being always-lower than full load speed; making the crane defi'nitel'y slower in its operation.

To raise the'stability limit ofthe series motorat weakened-field, thecommon practice with all the renowned control manufacturers is to introduce a permanent resistance step in series with the'armature duringlowering only. Addition of thisresistance in'its' turn reducesthe voltage impressed on the armature, and this reduces the"- light hook lowering speedof' the hoi'stto a greater" extent.

It istheobject of this invention to design acontrol circuit, provided with a simple relay, such that the light hooklowering speed" can be made almost as high as the full load lowering speed and at the same time increase the stability of the motor operation while handling heavy loads at high speeds.

According to this invention in the control circuit of the hoist panel, I introduce a double coiled relay, one of the coils being connected in series with a variable resistance across a resistance step in the armature loop, the other coil being connected directly across the resistance in the armature loop combined with the line resistance, 1. e., (resistance common to the armature and the series field loop), said relay being adapted to control two contactors, one normally open and the other normally closed, said two contactors being adapted to control the speed regulating resistances in the armature and the series field circuits such that high lowering speeds are obtained both while lowering light hook or heavy overloads without causing any instability in the motor.

The normally open contactor is connected across a resistance step in the armature loop and the line resistance (resistance common to the armature and the series field loop).

The normally closed contactor is connected between the resistance in the armature loop and the resistance in the series field. During lowering of empty hook and the pre-set values of light loads, the normally closed contactor remains energised keeping its contacts open while the normally open contactor closes only on the high speed points of the master switch; when the normally open contactor closes, the resistances get so connected that maximum resistance is introduced in the series field circuit giving maximum speed.

With overhauling loads in the high speed points for the master switch, the normally closed contactor gets deenergised closing its contacts while the normally open contactor does not get any power and remains open, with the result that the series field of the motor is strengthened while the line resistance remains in the circuit. Thus proper dynamic braking action of the motor is obtained at the same time increasing the stability of the motor.

For a better understanding of the invention, reference may be made to the following description taken in conjunction with the accompanying drawing wherein the figure is a schematic illustration of a control system wherein the features pertaining to my invention are introduced.

In the drawing an electric motor of the direct current series type is shown provided with an armature (ARM), commutating fields (CF) and series fields (SF), which may be suitably coupled to the operating mechanism of a crane hoist. Since the construction of the crane hoist mechanism and the manner of installation of the motors, as well as that of mounting of the electromechanical brakes are well known, it is not deemed necessary to illustrate them. Notation (SB) denotes the series type operating coil of the electromechanical brakes which is mounted on the motor shaft extension (not shown).

A master switch (MC) which a number of cam operated contacts is provided to control the operation of the motor. The contact segment H is disposed to engage the contact finger i so that as long as the two are engaged, the contacts are closed and the circuit is made complete. Thus the contacts 2 of the master switch MC close when they are engaged to the segment 22, and so on for other contacts in sequence.

Contactor Device Function Main negative line contactor, operating as a circuit breaker. Remains energised under normal condition.

Hoist directional contactor. Remains energised during hoisting only.

Lower directional contactor.

ing lowering only.

Lower series field contactor. Remains energized during lowering only.

Dynamic braking contactor (spring closed i. e., its contacts are kept normally closed by a spring, and open when the contactor is energised).

Armature accelerating contactor, closing on 3rd notch hoisting and 4th notch lowering with lighi loads.

Armature accelerating contactor closing on 4th notch hoist and 3rd notch lower.

Armature accelerating contactor closing on 5th notch hoist and 4th notch lower.

1st Field accelerating contactor opens on 3rd noich lower; remains always energized in the Ofl'position. as Well as during hoisting.

2nd field acceleration contactor, opens on 4th notch lower; remains energized in the 011' position, as well as during hoisting.

3rd field accelerating contactor, opens on 5th notch lower; remains energized in Oil position and (hiring hoisting.

Field accelerating contactor; spring closed; remains always energized, except when lowering overhauling loads. Contacts close on the 4th notch when 1A contactor does not go in.

Remains energized dnrlFA FUNCTION AND SETTING OF THE RELAYS Relay CRMagnetic time control relay This relay controls acceleration during hoisting and lowering under both normal loads and overloads. It provides adjustable definite time in energizing the contactor 2A. It is controlled over the master switch contacts '1. Its coil is kept energised in the off position of the master switch. It is deenergized on 4th notch hoist and 3rd notch lower. When the relay opens, its contacts close and give power continuity to 2A2 coil of contactor 2A, which then is free to be pulled in. Its timing can be set between 0.5 and 1.0 second as required. When the relay coil is deenergized, the relay allows its armature to release only when it has timed out.

The relay also controls the series brake operation during lowering. In lowering, if the controller is suddenly moved fast to its final position, this relay does not allow ZFA field weakening contactor to drop out until the relay has timed out. The ZFAZ coil of 2FA contactor gets its direct power over normally closed auxiliary contacts 2A4 of 2A contactor, which is controlled by the CR relay. Thus unless 2A contactor has pulled in, the ZFA contactor does not drop. This gives suificient time for the brake to release.

Relay AR-Accelerating relay This relay is similar to the CR relay. It controls 3A contactor both during hoisting and lowering to give proper sequence acceleration. It is set to time out in 0.5 second. Its coil gets de-energized when 2A contactor has pulled in.

Relay DRDeceZerating relay It is ascents l jlt is automatically delayed in timing outs This permits contactors: IE and 2E to remain energized longer. The motion is thus automatically brought -to slow speed by motor-dynamic braking action, which-is allowed to-act-for a-longer time.

When DR; has dropped out; IE contactor opens and 2E closes, and theser-ies brake sets in. Thus the jerks onthe crane hoistare reduced asthe brake has to handle less energy, because: the motion slows down sufficiently before the brake isreleased tohold the load;

Relay ALRe-Accelera-ted Zowering relay This is the relay'which has been introduced according totliisinvention. It is termed ALE i: e., accelerated lowering relay.

Therelay has two independent coils fitted to the same magnetic core, indicated as- ALR'Z and ALE-3. The coil A l-R3 is connected across the resistance step points R1 and A i. e. combined steps Me -PH. Thesecondcoil ALRZ is connestedacrcss-thenarmature loop resistance step points RI and- Bi. e., resistance I43 only; To provide for adjustment at site; the ALR2- coil is taken in series over a variable resistance (e).

The relay is provided; with a shuntmagnetic' circuit, which prevents the relay from pullingin again when heavy loads are being handled. A standard magnetic relayused for crane service with the above features is proposed, modified to give the characteristics described; Therelayhas oneset of normally open contacts ALRI which close when the relay is energized and is pulled in:

Operation. of the relay ALR When the motor is switched" on, the current fiowsoverthe resistance steps I43 and I I I in the same direction". The voltages impressed on the two coils of therelay areth'erefore in the same direction. The relay is therefore pulled in instantly;

However; during lowering, when the load on the hookis overhauling'li e:,.when the load tends to drive the motor; the motor then becomes a generator; The current through the armature then reverses its direction, whereas. the current through the series field circuit. remains in the same direction. Thus the direction of current in the resistance. step I43 is reversed i. e., the current flows from point B to point RI. The current through the resistance I-II still is in same direction, i. e., from point B to point A. As the'motor becomes a generator, the field current-now: issupplied' by its own armature. Depending on the load, therefore, the field current coming from the line and passing through resistance III back to the line is reduced. The effective voltage dropacross the resistance step is reduced due to two opposing currents in the branches (B-A) and (RIB). The voltage impressed on the ALR3" coil connected across points-Rf and A is thus-reduced, while the voltage across the ALRZ coil, acrossthe points RI and B is reversed. The relaythus becomes a differentialcompound-relay. The efiective voltage i's-therefore not able to holdthe armature pulled in. Depending on the relay setting, the relay can be made to drop out on any overhauling load, even from the 2nd point of the master switch.

Therelay thus works as a cumulative compound relay, during lowering for light loads, and differential compound relay' for overhauling heavy loads; The values of resistance steps III and I 43 being always in fixed ratio; a; single 6." relay can be-used'for all motor'sizes and for any desirable overhauling loads- 'Fz'mct'ion of reliryALR The relay ALR controls theoperation of the contactor IA. Its normally open contacts are connected across the holding resistance of the coil -of-contactor IA,- showmas Resistance him the control diagram. When the ALR' is energized, the holding'resistanceis shorted over- ALRI contacts. The relay thus-controls the operation of the contactor IA;

Having" thus 1 explained the-functions of various contactors-and relays, the operation of the-controller is explained briefly as follows. In the diagram only the points relevant to the description of the invention have been shown in order tomake the" explanation as simple and the diagram as clear as possible.

Hoisting'cycle As soonas thecontrolpower is putonby closingthemainswitchand the control knife switch, the power circuit becomes complete from the power line. ILover the contacts l of. the master switchMC, over theoperating coil M2 or the main contactor M to thenegativepower line 2L. The contactor M isenergized andits mainlcontacts MI close. When M- closes, its. coil- M2 getspermanently connected over its ownauxiliary contacts M4, whichalso close-when thecoil is. energized; The contactor. M- thusremains energized-for all positions of the master switch MC. Whenthe master switchis moved to the 1st position in the hoist direction, its contacts ii engage with the segment 33: and thusget closed. This energizesthe coil HZ of. contactorH whiehcloses its maincontacts HI; Power then fiowsfrom vthe'positive line IL -over the closed contacts Hl .then.passesover thetrolley cross barcollector CB2, over the series coil'SB of the electromechanical brake, through the series field .SE, through-the motor commutatingfieldCF. and themotor armature. (ARli/I)v over the. trolley cross-bar collector CA2, through the resistance I4,-then over the resistance step III, over the main contacts MI, back to the negative line 2L. The motor is thus connected as a straight seriesmotor over the control resistances and r0- tates in the hoisting direction asthe series: brake also gets energized'and releasesitsholding shoes. To increasethe motor speed, themaster switch is moved further to points 2, 3, 4 and: 5,.which in sequence energize the contactors 2E, IA, 2A and 3A all-owing the control resistances I2, I4 and I II to be shortedfrom the motor circuit. Onthe 5th point of the master switch, the motor gets connected directl -onto the power lines Ill-and 2L which gives the maximum hoisting, speed.

Lowering. cycle.Function. of the'relay ALR and contactors 1A and, 4FA

It is the object of my invention todesign a circuit, which'gives an automatic uniform accelerationof the'crane hoist serviceduring the lowering cycle, whatever'the magnitudeof the load handled bythecrane, from empty hook to the maximum permissible overloads, without disturbing thesta-bility of operation, i. e., without creating any-'run-away conditions inherent in the direct current series" wound motor, when working with highly weakened fields at high speeds during lowering of heavy loads.

The invention lies in the control circuit design which is explained as-follows:

(i) Lowering with light loads Moving the master switch to the lowering direction, on the 1st point, contacts 2 engage with segment 22, and contacts 4 engage with segment 44 which makes the circuit to coils L2 and IE2 of contactors L and IE respectively. The contactors are thus energized and contacts LI and IEI close. The current thus flows from the positive line IL over the closed contacts LI, over the trolley crossbar collector SAI. After CAI, the current divides itself into two branches, one branch fiowing along the point AI through commutating field CF and the armature ARM and the resistances I4; while in the other branch, the current goes through the motor series fields SF, the brake coil SB and the resistances I I. At the point B, the branch currents combine together and flow over the resistance ill, and back to the negative line 2L over the main contacts MI of the contactor M. Thus during lowering the motor armature and the motor series field are separately excited, connected in parallel, which gives the motor a shunt characteristic while lowering. In the series field circuit, the coils IFA2, 2FA2 and 3FA2 of contactors IFA, ZFA and 3FA remain energized over the master switch contacts I0, I I and I2 as those master contacts remain engaged with their segments I00, H and I20. The current through the motor series field is thus limited by the resistances H2 and III. The resistances are so selected that the motor field gets its full field current. The series brake also gets sufficient current to release its holding shoes instantly.

The contactors IA, 2A, 3A and 2E are, however, not excited. The armature thus remains shunted over the resistance I2, points EI--E2, as 2E contacts remain closed. The armature circuit has therefore full resistance I4 in circuit. The voltage across the armature is such that the hook is lowered at a very slow speed, called creeping speed.

When the master switch is moved to 2nd point lower, its contacts 5 close, giving power to coil 2E2 of contactor 2E which pulls in and opens normally closed main contacts ZEI. The armature shunt circuit is thus opened, causing the motor to increase in speed.

When the master switch is moved to the 3rd point lower, its contacts I and I0 open and contacts 8 close. Contacts II), which control the coil I FA2 of contactor IFA, thus break the power to that coil and the contactor IFA drops out, which opens main contacts IFAI in the series field circuit. This introduces resistance step H5 in series with the field; the field current is thus reduced, accelerating the motor further.

Master switch contacts I while opening deenergize the coil CR2 of relay CR. The contacts CRI of the relay close after a time delay of about 0.75 second (which is the time setting of the CR relay). As the CR relay times out, its contacts CRI close, giving circuit to the coil 2A2 of 2A contactor. As master switch contacts 8 close on the 3rd notch, 2A2 coil of contactor 2A is energized, because auxiliary contacts IFA3 of contactor IFA are already closed, 2A contactor pulls in closing its main circuit contacts 2AI. This short circuits the armature resistance step I42 giving higher voltage to the motor, which thus rises in speed still further. The contactors IPA and 2A are so interlocked, that IFA always opens first, after which 2A closes. Thus field is weakened first, after which the armature is strengthened to give rapid and smooth acceleration.

When the master switch is moved to 4th point lower, its contacts 6 and 9 close, while the contacts II open. Opening of master contacts II, de-energizes the coil 2FA2 of contactor ZFA, because auxiliary contacts 2A4 of contactor 2A are already open. This opens ZFAI main contacts in series field circuit, introducing resistance H4 in circuit. The field current is thus reduced, which accelerates the motor to the next higher speed step.

Closing of master switch contacts 9 gives connection to 3A2 coil of contactor 3A, which is also controlled by ARI contacts of relay AR. As 2A contactor is already pulled in on the 3rd point of the master switch, its auxiliary contacts 2A3 which control AR2 coil of relay AR, open and deenergize the relay coil ARZ. The relay AR then drops closing its contacts ARI, permitting 3A contactor to pull in. 3AI contacts of contactor 3A when closed, remove resistance step I4I from armature circuit. The motor thus moves faster.

Closing of contacts 6 of the master switch gives power to coil IA2 of contactor IA, as the auxiliary contacts 2FA3 of contactor ZFA are already closed because ZFA contactor is open, and the auxiliary contacts 4FA3 of contactor 4FA are also closed as 4F'A contactor remains energized. As motor is lowering light hook, the voltage drop across the resistance II I due to the field current flowing through it is of such a value that the relay ALR remains energized with its contacts ALR I closed. This permits the contactor IA to close its main contacts IAI. Once IA contactor goes in, it is held in as the circuit is completed over the resistance b. Closing of IAI contacts of contactor IA connects the motor directly to the main lines. However, in the field circuit, the resistances I43 and III get connected in parallel over IAI contacts. The resistances are so adjusted, that the series field has maximum resistance connected in series with it, while the armature is connected direct to the lines. This gives maximum lowering speed for light hook, depending on the resistance values selected. It should be noted that the contactor coil 4FA2 of contactor 4FA remains permanently energized, when IA is energised and its auxiliary contacts IA3 close, giving holding power to 4FA2 coil of the contactor 4FA.

(ii) Lowering with heavy loads When there is overhauling load on the hook, the load begins driving the motor, which thus automatically becomes a generator, producing the counterbalancing dynamic braking action. When the magnitude of the load is sufiicient, the relay ALP. becomes a differential relay as explained before. When the load to be lowered is of the magnitude for which ALR relay is set to open, the relay ALR will get de-energized already on the 3rd point of the master switch and drop out, opening its contacts ALRI. The behaviour of the control up to 3rd point of the master switch is the same as for light hook duty. However, on the 4th point of the master switch, 3A contactor gets energized, but IA contactor does not go in, because ALRI contacts of the relay ALR are open which introduces the resistance 1) in series with the coil IA2 which prevents the coil to have suificient power to operate contactor IA. At the same time, the auxiliary normally closed contacts of 3A4 of contactor 3A which give power to 4FA2 coil of contactor 4FA open out, when 3A contactor closes. As however IA contactor also remains open which keeps its aux- 9 'zlliary :contacts [as :to remain eopen. This ;opens out the power :circuit so'f 0011 IFAZ of "contactor liF n-whichzgts de-;energized;and drops out, clos- Zing "its :main znormally :closed contacts AFAI.

ClosingpfA-FAI contacts ithen parallelsthe resistances I43 and 2IIi2. 'Thusthe field is again weakened to '.-its maximum avalue, however, -the :resistances :are so adjusted that a stable full "load Zlowering speed .of lbetween "180-200 per cent is obtained.v

"The values :ofthe :resistance "steps can be. given in :per -unit"values. One :per unit resistance is defined as the quotient of rated motor voltage divided by the rated full load motor :current. "Thus 'for light loadstotal field resistance of 2.15 per unitccan be introduced awhile for :overhauling "loads, that vzilue becomesfl'BS per unit. For 230 volts, therefore, the total field current 'for maximum speed at light hook is-about45 per cent, 'and that for-heavy overhauling loads is about 54 per cent. This would give about the same maxi- :mum hook 'speeds, both for light hook and full load on the'hook.

Increased :stability at lowering heavy loads On overhauling loads, it is observed that IA .contactor does1not go in, while 4FA contactor also drops out. It .will'therefore be observed'that the resistance I I' becomes connected to the negative I'line, asa series resistance'both to the armature and the series field.

With heavy loads on the hook, particularly on the last points ofthe-master switch, the armature necessarily takes a heavy current; it supplies its own field current and 'also feeds cur- :rent hack into the line, :the distribution of the current depending on the working conditions of the system-at the time. Theiarmature reaction therefore becomes more predominant and tends to weaken the motor field, "which may produce unstable condition at -any time.

Addition 'of the series resistance in the line thus re-distributes the current distribution between the series field of the motor and the main power lines. It tends to force more current through the series field circuit and depending on the resistance value of resistance II I in series with the main line, the current fed back to the power lines can be controlled during lowering of very heavy loads.

The armature reaction demagnetising efiect is thus better counterbalanced by the series resistance in the line circuit, which increases the stability of motor operation at reduced field. currents with heavy loads, by forcing more current through the series field as the load increases thus strengthening the generator counterbalancing action of the motor.

Ihe main features of the control system herein described are thus given below:

(a) The main circuit as herein devised gives equal number of speed steps both in hoisting and lowering directions.

(1)) The main circuit utilises the same number of contactor and relay devices, as in any other well-known system: however, the sequence of operation of the contactors is so adjusted that a contactor operates only once during a complete cycle of operation.

The main circuit gives distinct resistance steps, both in armature and series field circuit, independent of each other. The resistances can thus be adjusted either in armature for hoistin speed adjustment or in series field for lowering zspeeds adjustment, without one appreciably dis- 'ing between contactors.

(f) The maincircuit permits increased speeds of the crane hoist uniformly from empty hook to heavy loads, without introducing any extra resistance step or extra contactor device.

(9) The main circuit-permits increased stability-of motor operation athigh speeds with heavy overhauling leads, by automatically adding a resistance in series with the "line with overhauling hook loads.

(h) The relay ALR works automatically as compoundadditive relay, was 'a compound 'differential relay, depending on the load on the hook of the crane hoist.

(i) The relay ALR serves for all the motor sizes, normally used for cranes 'upto maximum capacity.

(i) Relay ALB, permits high lowering speeds both for light hook 'andheavy overhauling loads. (is) The relay ALR gives maximum stability feature to the whole control while working at heavy overhauling loads 'by controlling a resistance in the main line circuit of the motor. Having now particularlydescribed and ascertained the nature :of my said invention and in "what manner-the same is 'to "-be performed, I 'declare that what I claim is:

1. A magnetic controller for electrically op- -erated travelling-cranes employing an electrical circuit combining 'a -'('iirect current motor, a

source of energy, and means of connecting the motor to operate as a series motor while raising a load, and connected to operate as a shunt motor while lowering a load, incorporating a shunt relay provided with two similar coils, one main operating coil connected to compare the algebraic sum of the voltage drops across a resistance in the motor armature circuit together with a resistance connected to the main power line and in series with the parallel circuits formed by the motor armature and the motor field during lowering of the load, and the other coil to increase the sensitivity of the operation of the relay connected in series with a variable resistance, the two coils together being connected across the resistance in the motor armature circuit, the said shunt relay combining the two coils operating as a compound relay during the hoisting of the load, and also during the lowerin of a light non-overhauling load, and operating as a difierential relay while lowering an overhauling load, the said relay being so connected that the same relay serves to control motors of different horsepowers and ratings, the said relay being adapted to control two magnetic contactors, one contactor with normally open main contacts and the other contactor with normally closed main contacts, the said two contactors controlling the speed regulating resistances in the armature and the field circuits of the motor, said resistances being so proportioned that the lowering speed with no-load on the hook becomes as high as the lowering speed with full load on the hook, this operation of the contactors simultaneously adding a resistance in the main power circuit which diverts more current to the field of the motor thus automatically balancing the motor speed and increasing the stability of the motor, enabling the crane to handle the maximum overloads it is capable of, without any danger of the runaway conditions due to natural instability of a direct current mill type series motor under lowering conditions.

2. A control system for hoisting service employing a direct current motor connected to a source of energy and worked as a straight series wound motor while hoisting and as a shunt wound motor while Wering, including means for automatically controlling the regulating resistances over two contactors, one contactor having normally open contacts being connected across the combination of two resistance steps, one step being in the armature circuit and the other step being in series with the power line and the parallel circuit formed by the armature and the field during the lowering cycle; while the other of the tWo contactors, having normally closed contacts being connected between the resistances in the armature and the field, the said two contactors controlling the motor field current and the armature-back-electromotive force so as to give always maximum lowering speeds limited to the same values with the crane hook either empty or with full load hanging.

3. A control system for hoisting service employing a direct current motor connected to a source of energy and worked as a straight series motor while hoisting and as a weakened field shunt motor while lowering, providing a line resistance to be introduced automatically in series with the power line and the closed loop formed by the armature and the field combined, increasing the stability of a weakened field direct current motor handling heavy loads, the

said resistance being controlled by a contactor with normally open contacts, adapted to be introduced only during lowering of loads which are overhauling and being automatically shorted over the said contactor while lowering a light hook or a non-overhauling load.

4. A control system for hoisting service, combining a direct current motor, a source of energy, and means of connecting the motor to the source of energy to operate as a series-wound motor for raising a load and connected to operate as a shunt-wound motor for lowering a load, means for automatically varying the field strength of the motor when operating as a shunt wound motor to control the speed to the same maximum value when lowering a light hook as when lowering a full load on the hook, and simultaneously automatically introducing a resistance in series between the source of energy and the loop circuit formed by the motor armature and the motor field, and thereby increasing the stability of the motor even while lowering heavy overloads, and introducing during lowering a loop which is always closed in the armature circuit without any making or breaking contact points, and further automatically introducing a second dead closed circuit over normally closed contacts of a deenergized contactor, thus completely eliminating the chances of an open circuit in the armature loop which causes unmanageable over-speeding when heavy loads are lowered at high speeds.

KAI-KOBAD S. KUKA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,781,792 Rodman Nov. 18, 1930 1,819,465 James Aug. 18, 1931 

